The present invention relates to a sheet feeding and separating device for image forming equipment.
An electrophotographic copier, facsimile apparatus, printer or similar image forming equipment includes a device for feeding recording sheets one by one to an image forming station. It is a common practice to implement such a sheet feeding device with a friction type system using a pick-up member in the form of a roller or a belt. The roller or belt is made of rubber or similar material having a great coefficient of friction. Although friction type sheet feed system is simple in construction, it cannot feed sheets stably over a long period of time. This stems from the fact that the pick-up member has to be pressed against the sheet surface by biasing means such as a spring to achieve a great frictional force, and the pick-up member made of rubber or similar material has the coefficient of friction of the surface thereof changed by aging and by environmental conditions.
A suction type sheet feed system is also available which generates vacuum by the suction of air to thereby transport a sheet. While this type of system is more stable than the friction type system, it is not feasible for office and home use since it produces great noise due to the suction of air and increases the overall dimensions of the device.
On the other hand, it is likely that the friction type sheet feed system pays out two or more sheets at the same time. It is, therefore, necessary to use a device for separating one of such sheets from the others. This kind of device is often implemented with a friction pad made of rubber or similar material having a great coefficient of friction or a member rotatable in a direction opposite to the intended direction of sheet feed, i.e., a counter-feed direction. The friction pad or the rotatable member is held in pressing contact with the pick-up member or a transport roller located downstream of the pick-up roller. The coefficient of friction between the pick-up roller or the transport roller and a sheet, the coefficient of friction between the friction pad or the rotatable member and a sheet, and the coefficient of friction between sheets are sequentially reduced in this order. In this configuration, when a plurality of sheets are paid out together, one of them which contacts, for example, the pick-up roller is separated from the others and fed out. When only one sheet is paid out by the pick-up roller, it is fed out against the friction between, for example, the friction pad and the sheet. This type of separating device, however, cannot surely separate sheets when the sheets stacked on a cassette are stuck together due to the absence of air layer therebetween or static electricity or when the sheets are nappy and twine together on the surfaces thereof.
U.S. Pat. No. 2,459,773 (Japanese Patent Laid-Open Publication No. 7847/1981) discloses a sheet separating device using a transport roller and a reversible arresting roller in place of the above-stated friction pad. The transport roller and arresting roller are located downstream of a pick-up roller and on both sides of a sheet transport path. The transport roller is rotatable in the direction of sheet feed while the arresting roller is subjected to a constant torque in the counter-feed direction. When the transport roller and arresting roller are directly pressed against each other or when only a single sheet is paid out by the pick-up roller to between the transport roller and reversible roller, the arresting roller does not slip on the transport roller or the sheet. This is successful in eliminating the wear of the rollers, the decrease in the coefficient of friction, paper dust ascribable to operations, and the decrease in the coefficient of friction due to such paper dust. However, since this type of separating device still relies on the difference between the coefficient of friction between the rollers and the sheet and the coefficient of friction between the sheets, it cannot surely separate one of a plurality of sheets paid out together from the others when sheets stacked in a cassette are stuck together for the previously stated reasons.